1. Field of the Invention
The present invention generally relates to telecommunications networking technology. More particularly, the present invention relates to a scalable network device, such as a switch or a multiplexer (e.g., DSLAM, ATM switch).
2. Description of Related Art
At its most fundamental level, a telecommunications system should provide service between at least two end users with a minimum level of quality that is acceptable for the kind of traffic that is being transmitted between these end users. End users have different needs from the telecommunications system because of the types and volume of traffic between them. One end user could telephone another user for voice communication. Another end user could use a computer and a modem to send an email to another user for data communication. Additionally, another end user could use a computer and a modem to access any number of Web pages on the Internet. Depending on the type of data (e.g., text, graphics, audio, video) that is being accessed, bandwidth requirements will vary. Thus, one end user may have very basic needs with minimal bandwidth and lots of delays, while another end user may have stringent requirements for lots of bandwidth with minimal delays. How can a telecommunications system, and its components, be designed to accommodate the various uses and needs?
Currently, the telecommunications infrastructure comprises network nodes (e.g., ATM switches, routers, other central office switches) that are coupled together. At the edge of this infrastructure are edge nodes (e.g., DSLAMs) for providing telecommunications service to subscribers (e.g., home users, business users, ISPs) via some local loop technology (e.g., ISDN, DSL). As known to those skilled in the art, local loop (or loop) refers to the interface (typically wires) between the subscriber and the telephone company's central office. Some of these network and edge nodes can be combined to create corporate networks and Intranets. These pieces of node equipment are usually installed and located in central offices as well as corporate offices. Communication between or among subscribers is possible through these network and edge nodes which run various communication protocols.
Although the bandwidth on the telecommunications backbone (e.g., DS3, OC3, STS3, or OC12) may be sufficient for all of the various types of traffic generated by subscribers, the bandwidth on the loop side is limited for today's traffic needs. Historically, analog dial-up with analog signaling was more than sufficient for most traffic. Even the digitization of voice, which requires approximately 4,000 Hz bandwidth, posed no problem since the telephone companies provided 64 Kbits/sec of bandwidth per channel. However, with the rise of the Internet and the increased interest in digital video and audio services, earlier modems were sorely lacking as speeds were as low as 1,200 bits/sec (V.22) to 33.6 Kbits/sec (V.34+) to today's 56 Kbits/sec (U.S. Robotics/3COM and Lucent/Rockwell). Even at 56 Kbits/sec, these modems are still too slow to fully capture digital audio and video services to the end user's satisfaction. Even if modems could use all of the available 64 Kbits/sec bandwidth provided by the telephone company, users would still be dissatisfied for the latest audio and video digital services. To give the appearance that more traffic was being passed through the lines than was actually being transmitted, several companies developed and implemented compression techniques.
For some time, even before the onset of the Internet, several loop technologies were slowly developed for this market. One such loop technology is ISDN (at 128 Kbits/sec), which failed to deliver on its promise of better digital service because of the high cost of ISDN equipment and the higher cost of upgrading central office hardware and software. Furthermore, ISDN equipment was difficult to set up and make compatible with existing equipment because of poor diagnostics. The rates set by the public utilities commissions of various states are also very high. As of this writing, however, ISDN still has not disappeared and some service providers are offering ISDN service.
The traditional forms of leased line technologies, including T1 (at 24 channels of 64 Kbits/sec each for a total bandwidth of 1.544 Mbits/sec), fractional T1 (where users can get as many T1 channels as desired), and T3 (at 44.736 Mbits/sec or 28 T1 circuits), were also, of course, available. However, their prohibitively high expense makes these line technologies less attractive to the average subscriber. Recently, the pace of loop technology development has picked up due to the rise and proliferation of Digital Subscriber Line (DSL). Other competing technologies include Cable (CATV) Data Networks and Modems (or cable modems) and fiber optics.
At this point, however, DSL appears to be the front runner for the broadest application across the telephone companies' residential and small business subscriber base. However, DSL may not eliminate other technologies (e.g., T1, T3, ISDN, and cable modem) as the various telephone companies will make these technologies available to those who want them and can afford them. Various DSL technologies are available and they differ in technical operation rather than application. However, ADSL is receiving the most attention for loop technology in general and Internet access in particular. The entire xDSL family is provided below in TABLE A:
TABLE AxDSL FamilyNameData RateModePhysicalHDSL1.544MbpsSymmetricTwo wire pairsHDSL22.048MbpsSymmetricOne wire pairSDSL768KbpsSymmetricOne wire pairADSL1.5 to 8MbpsDownOne wire pair but 18 Kft max16 to 640KbpsUpRADSL1.5 to 8MbpsDownOne wire pair but can adapt16 to 640KbpsUprates to line condition changesCDSLUp to 1MbpsDownOne wire pair and require no16 to 128KbpsUphardware at CPE premisesISDLISDN BRISymmetricOne wire pairVDSL13 to 52MbpsDown1 to 4.5 Kft max and needs1.5 to 6MbpsUpfiber feeder and ATM
The first column, “Name,” identifies the type of xDSL technology. Refer to the GLOSSARY to obtain the full name from the acronym. The second column, “Data Rate,” is the typical maximum rate at which data transfers are accomplished for the various modes (see third column, “Mode”) for each xDSL type. While ADSL, RADSL, CDSL, and VDSL are non-symmetric (the downlink CO-to-subscriber rate is usually faster than the uplink subscriber-to-CO rate), HDSL, HDSL2, SDSL, and ISDL are symmetric. The fourth column, “Physical,” provides a brief description of the distinctive physical layer requirements or properties for each of the xDSL types.
To make DSL work at a central office, a DSL access multiplexer (DSLAM) is needed. However, current DSLAMs are limited. Although the various types of DSL technologies will be implemented, most existing DSLAMs cannot support multiple DSL technologies; that is, a given DSLAM can only support one DSL technology. This limits what the telecommunications providers are willing to do to expand DSL throughout their respective network. Given the shortage of space in central offices today, telecommunications service providers may be unwilling to purchase multiple DSLAMs for all the different types of DSLs that are available. In other cases, these telecommunications service providers may only deploy certain DSL types to be supported by the limited DSLAMs.
Compounding the problem of existing DSLAMs being tied to a particular DSL type is the fact that some existing DSLAMs are also tied to a particular modem modulation technique, either CAP or DMT. A DSLAM that is compatible with both modulation techniques would be desirable.
Another problem with existing DSLAMs is the nature of the virtual connections supported. In order for any two subscribers to communicate, a virtual connection must be set up between them. The end users and the network nodes predefine and maintain a virtual circuit and virtual path through the packet-switched mesh-type network that the end users believe to be a dedicated physically connected circuit. The actual path, however, may change due to routing around down or busy connections through the ATM/router maze-like network. Regardless of the path, the packets are transferred, in order, over a specific path and arrive at the destination in order. Two types of virtual connections are known—permanent virtual connection (PVC) and switched virtual connection (SVC). Existing DSLAMs do not support SVC because SVCs require signaling logic in the switches and end systems. Existing DSL modems, which are typically located at the end users' locations, do not support SVC signaling.
A permanent virtual connection, or PVC, is a fixed circuit that is defined in advance by a public carrier or a network manager. Thus, all data traffic between two end systems that signed up for a PVC follows a predetermined physical path that makes up the virtual circuit. PVCs have traditionally been tailored for subscription-based services and are typically maintained for a long period of time. A PVC is permanently programmed into a network for continuous use (or as long as the subscriber desires) between end users for multiple sessions. Typical applications include multiple geographically disparate LANs tied together via a PVC to create a WAN so that data traffic can be supported between users of the two LANs.
The fixed nature of the PVC circuit removes setup and disconnect overhead but takes time to initially establish because each end user and node along the path have to be configured with all the PVC parameters by the network management system (e.g., carrier), which may also involve human intervention. Typically, however, the network administrator uses a console which provides a topology of the network and the network administrator has to set the proper routing parameters node by node. This is a tedious and time-consuming process.
The physical and permanent nature of PVCs is a major drawback. If any physical transmission line among the network nodes along the preprogrammed PVC path fails (e.g., the line is physically cut), this PVC-based telecommunications service between the calling parties may not be remedied in a timely fashion.
In contrast, a switched virtual connection, or SVC, is a temporary virtual circuit that lasts as long as the session between end users lasts. If a carrier supports SVCs, it can set up and disconnect SVCs on the fly using SVC signaling. Once the communication for a particular session is complete, the virtual circuit is disconnected. Thus, alternate paths can be set up among sessions between the same two communicating end users.
While PVC is statically set up and torn down by the network management system (e.g., carrier) in a very inefficient and untimely manner, SVCs are set up and torn down dynamically by the SVC signaling equipment at the ATM end users' locations using a UNI signaling protocol. Thus, the SVC parameters are provided to the switches along the path for automatic configuration by the signaling protocols in a distributed fashion. However, as stated above, current DSLAMs do not support SVC signaling primarily because the loop side of the DSLAM is PVC-based. To provide flexibility to the telecommunications service providers, a DSLAM that supports SVC signaling even though the loop side is PVC based is needed. In effect, despite the lack of SVC signaling from the loop side, a DSLAM that can provide “proxy” signaling, i.e., logic in the DSLAM that provides signaling to other network nodes to give the appearance that SVC signaling came from the loop side, is desirable.
Another problem with existing DSLAMs is that they do not support Quality of Service. When a subscriber signs up for some telephone service at a particular Quality of Service, the telecommunications service provider physically sets up the connection. As known to those skilled in the art, Quality of Service (QoS) is a measure of the telephone service quality provided to a subscriber. With respect to ATM, QoS refers to the bundle of parameters that are associated with, at the very least, bandwidth, timing, and cell error/loss. In particular, the ATM Forum defined certain parameters in its UNI ATM Performance Parameters including bandwidth, cell error ratio, cell block ratio, cell loss ratio, cell transfer delay, mean cell transfer delay, and cell delay variability.
Another problem with existing node equipment is their undesirable blocking effect. As known to those skilled in the art, “blocking” and “non-blocking” have many definitions. The typical usage of the term “non-blocking” refers to the situation that when data at a particular bandwidth comes in to one of the input ports of the node, the output ports of the node should provide that data at the same bandwidth. In other words, the node should not block that data. Simply stated, blocking means that a call cannot be completed. Blocking usually occurs when the switching or transmission capacity is not available at that time. The blocking phenomenon typically arises because a telecommunications service provider oversubscribed its services; that is, the total number of subscribers supported by the switching equipment cannot have their full share of bandwidth and services during times of peak usage.
Understandably, no telecommunication service provider designs its system for complete non-blocking capability. This would be too expensive as more and more sophisticated equipment and circuits are needed to provide more and more bandwidth for the given number of subscribers. The telecommunications service provider makes a trade-off decision—what is the provider (and its subscribers) prepared to pay versus what is the provider (and its subscribers) prepared to tolerate in terms of blocked calls or data? More and more telecommunications service providers are willing to make this decision in favor of higher cost to assure its subscribers a higher grade of service. Because oversubscription is a common practice in the industry, most DSLAMs provide blocking, thus eliminating all the bandwidth gains achieved while the data makes its way to these DSLAMs.
Despite these problems associated with blocking and oversubscription, most industry efforts have not been focused on prioritizing the incoming data into service requirements. As mentioned earlier, the various data received by a DSLAM may have specific service requirements (e.g., bandwidth, time synchronization) which may be affected by the blocking phenomenon. To overcome this problem, Quality of Service (QoS) could be implemented to “fairly” prioritize the various data received by the DSLAM so that some data can be serviced before other data, while at the same time, ensuring that all the data received are somehow “fairly” serviced. As known to those skilled in the art, an unexploited feature of ATM is the ATM Forum's ATM service categories. Five different service categories have been defined, including Constant Bit Rate (CBR), Real-Time Variable Bit Rate (rt-VBR), Non-Real-Time Variable Bit Rate (nrt-VBR), Available Bit Rate (ABR), and Unspecified Bit Rate (UBR).
CBR refers to Constant Bit Rate and it is used primarily for those types of data where end systems expect some time synchronization. CBR data requires some predictable response time and a static amount of bandwidth continuously available for the lifetime of the connection. The peak cell rate determines the bandwidth. Typical applications include video conferencing, voice telephony services, and on-demand services.
VBR refers to Variable Bit Rate. It can be either real-time (i.e., rt-VBR) or non-real-time (i.e., nrt-VBR). Real-time VBR is used for connections that transport variable bit rate traffic that relies on time synchronization between end systems. Real-time VBR is dependent on peak cell rate, sustained cell rate, and maximum burst size. Typical applications that use rt-VBR are variable rate compressed video.
Non-real-time VBR is used for connections that transport variable bit rate traffic that does not rely on time synchronization between end systems. However, nrt-VBR requires some guaranteed bandwidth or latency. Typical applications include frame relay interworking. A vast majority of current DSLAMs also do not support extensive Quality of Service (QoS). Accordingly, telecommunications service providers cannot build end-to-end ATM networks over DSL from the subscriber's premises through a carrier ATM backbone network.
ABR refers to Available Bit Rate which is used for connections that transport variable bit rate traffic for which end systems do not expect time synchronization. Beyond a minimum cell rate, the end systems also do not require guarantees of bandwidth or latency. In effect, ABR is generally designed for those traffic that are not time sensitive and expects no service guarantees (beyond minimum cell rate). Flow control mechanisms are used to dynamically adjust the amount of bandwidth available to the user. The peak cell rate determines the maximum possible bandwidth. Typical applications may include TCP/IP and other LAN-based traffic.
UBR refers to Unspecified Bit Rate is used for connections that transport variable bit rate traffic which does not rely on any time synchronization between end systems. Unlike ABR, flow control mechanisms are not used to dynamically change the available bandwidth that is available to the user. UBR is used primarily for those types of traffic that are very tolerant of delay and cell loss. Typical applications include TCP/IP and store-and-forward traffic like file transfers and email across the Internet LAN and WAN environments.
As mentioned above, a DSLAM that incorporates QoS to prioritize the various types of incoming data would be desirable. With such QoS support, a DSLAM would be better equipped to handle all the different service requirements of the incoming data.
Accordingly, a need exists in the industry for a system or method that addresses problems raised by currently known DSLAMs and ATM switches.